(1) Field of the Invention
The present invention relates to an aqueous pigment ink composition. Particularly, it relates to an aqueous pigment ink composition suitable for ball-point pens.
(2) Description of the Related Art
In a conventional aqueous pigment ink for a ball-point pen, a pigment is dispersed as a coloring agent, and water or a water-soluble solvent such as glycol or glycerin is used as a solvent which is the main component for inhibiting the ink from drying (non drying properties) at the tip of the ball-point pen. However, such a conventional ink is poor in lubricating properties, so that frictional resistance between a ball and a ball seat is large when the ball is rotated at the time of writing, and thus, there are the following drawbacks. That is, the ball seat is worn and the ball retracts, and the ball is loose or the flow of the ink is not uniform, with the results that the writing performance noticeably declines. For the purpose of removing these drawbacks, it has been attempted to add a lubricating improver such as an unsaturated fatty acid or potassium oleate.
On the other hand, Japanese Patent Application Laid-open No. 63-234073 discloses an ink for a ball-point pen containing a dye and an adduct of polyglycerin with ethylene oxide, and Japanese Patent Application Laid-open No. 55-106273 discloses an aqueous ink composition for a ball-point pen containing a dye, an adduct of sorbitol or glycerin with polyoxyalkylene and a fluorine surface active agent. In both the publications, it is described that these inks for the ball-point pens are excellent in lubricating properties and have a smooth writing performance. However, when the dye is used together with the adduct of the alkylene oxide, the lubricating properties are excellent but there is a drawback that the ink tends to blur. Furthermore, Japanese Published Patent Application No. 62-10547 discloses a recording liquid suitable for an ink jet recording system containing an adduct of glycerin with alkylene oxide. However, when the recording liquid is used for the ball-point pen, there are drawbacks that the lubricating properties are not always satisfactory and that the dispersion of a pigment is not good.